1. Field of the Invention
The present invention relates to a method to manufacture a semiconductor device.
2. Related Prior Art
In a process to etch a slab material such as a semiconductor wafer made of silicon carbide (SiC), or a semiconductor layer disposed on the slab material, a mask layer is first prepared on the slab material as an etching mask. The mask material preferably shows an etching rate substantially greater than that of the slab material, which is ordinarily called as the selective ratio, so as not to vanish during the etching. A metal mask generally shows a large selective ratio, namely, a considerably small etching rate. A Japanese Patent published as JP-2008-098456A has disclosed a method to etch the SiC wafer selectively by using a metal mask.
The metal mask may be preferably prepared by plating. Specifically, Coating a positive resist on a whole surface of the slab material first; then patterning the coated resist by an ordinal photolithography technique, finally, a metal may be formed on portions not covered by the patterned resist. Removing the resist, the metal mask may be obtained.
However, the positive photoresist inherently has a characteristic that an irradiated portion is removed by the development. Accordingly, when the positive photoresist is coated in thick, the photolithography process often shows, so called, the under exposure failure, which leaves a residual resist that strongly influences the quality of subsequently carried out processes.